Microphones are commonly used in vehicular applications to control vehicle telematics using speech recognition and to interface with mobile telephones. Conventional microphone circuits typically included a DC power supply for powering a digital signal processor (DSP), and an output amplifier for amplifying the signals from the DSP. The DC power supply and the output amplifier were coupled in parallel so that input voltages of about 5V were available to both the DC power supply and the output amplifier, and there was sufficient current to power both components.
Recently, however, automobile manufacturers have sought to reduce power consumption by the various circuits in automobiles, particularly in electric and hybrid automobiles, as current draw by these circuits reduces the operating mileage range per charge of the batteries. Accordingly, with respect to microphones, it is now desirable to limit the power available to microphones, particularly the current draw of such microphone circuits. However, in the conventional microphone circuits, the input current must be split between the DSP and the output amplifier. This results in too low of a current level to drive the DSP.
A VDA interface is commonly used in automotive systems for reasons of low cost, elimination of ground loops and the ability to use unshielded wiring in some implementations. The power limitation described above can particularly become an issue in a microphone with extensive analog signal processing powered by a VDA interface. In situations where a class-B amplifier output stage is used, a maximum efficiency of only about 30% for sine wave signals is possible which typically requires high amounts of supply current.